Conventionally, projection type image display devices such as projectors have been broadly used as one type of devices for efficiently acquiring a large-screen image. The projection type image display device is configured such that a compact spatial light modulation element, such as a liquid crystal panel, for forming an image corresponding to a video signal is irradiated with light emitted from a light source such as a lamp and that the optical image is enlarged and projected on a screen by using a projection lens. In the projection type image display device such as a projector, an extra high pressure mercury lamp having a high light emitting efficiency in a band of wavelength of visible light are typically used as the light source.
In recent years, not extra high pressure mercury lamps but LEDs which are semiconductor elements have been increasingly used as light sources of projectors. The LED projectors have characteristics such as low power consumption, little noise, and a longer lamp life, and the size thereof is small.
However, since the LEDs use spontaneously-emitted light from active layers which are light-emitting layers, the brightness of such light is not sufficient. Thus, attention has been placed on laser projectors each using a watt-class high-power semiconductor laser as a light source and being capable of exciting phosphors by the semiconductor laser to provide sufficient brightness of light in a visible range.
At present, semiconductor lasers using a nitride-based material are capable of emitting light such as visible light and ultraviolet light, and are demanded as semiconductor light-emitting elements suitable as light sources.
For example, if a nitride-based semiconductor laser is used as a light source for blue and green light, and a laser using an AlGaInP-based material is used as a light source for red light, a compact low-power laser projector can be provided.
Not only watt-class high-power operation but also operation for a long period of time of equal to or longer than 10000 hours under a high temperature of equal to or higher than 50° C. have been demanded for semiconductor lasers used as projector light sources.
In order to obtain high-power output from a semiconductor laser, occurrence of catastrophic optical damage (COD) that a resonator end face is melted and damaged by light output therefrom should be reduced. In order to reduce occurrence of COD, it is effective that a front resonator end face from which laser light is extracted is coated with a dielectric film having a low reflectance (AR) of equal to or lower than about 10% and that a rear resonator end face is coated with a dielectric film having a high reflectance (HR) of equal to or higher than about 90%. According to such a configuration, a light extraction efficiency from a front end side can be improved, a slope efficiency (the rate of light output increase to current injection) in association with current-versus-light-output characteristics can be improved, and an operating current value can be decreased to reduce heat generation of the semiconductor laser. Further, even if the semiconductor laser outputs laser light at a certain level, a light density is lower on the front end side in the resonator. As a result, the light output level can be increased without occurrence of COD.
Typically, in the case of semiconductor lasers using an AlGaInP-based material or an AlGaAs-based material, the semiconductor lasers having an end-face window structure have been broadly used in order to ensure the long-term operation of the high-power semiconductor laser requiring operation with a power of equal to or higher than several hundreds megawatts. In this structure, quantum-well disordering occurs by impurities diffused in part of a quantum well active layer in the vicinity of a resonator end face, thereby increasing bandgap energy. Accordingly, the region of the active layer in the vicinity of the resonator end face substantially becomes transparent to laser light. As a result, a decrease in bandgap energy due to heat generation caused by light absorption at the resonator end face can be reduced. This reduces light absorption at the resonator end face, and occurrence of COD can be reduced.
On the other hand, in the case of lasers using a nitride-based material, it is difficult to cause quantum-well disordering by impurity diffusion because of the characteristics of an InGaAlN-based material used for a quantum well active layer. For such a reason, an end-face window structure has not been currently used for such lasers.
At present, in order to reduce a light density at an end face in a ridge-shaped waveguide structure of a watt-class nitride-based semiconductor laser, the following waveguide structure is employed: the semiconductor laser has a wide ridge-shaped stripe with a width of at least about 8 to 15 μm; and an AR coating is formed on a front end face, and an HR coating is formed on a rear end face. This greatly expands the distribution of light in the waveguide in the horizontal direction. Accordingly, the light density at the resonator end face decreases, and the light output level without COD can be improved.
Note that, in the present specification, the direction perpendicular to a resonator direction and parallel to an active layer is referred to as a “horizontal direction,” and the direction perpendicular to the resonator direction and parallel to a normal direction of the active layer is referred to as a “vertical direction.”
Typically, in high-power semiconductor lasers having a straight stripe structure in which a ridge-shaped stripe width is maintained constant in a resonator direction, if the difference between reflectance Rf on a laser light emission end (AR) side and reflectance Rr on a rear end (HR) side is significantly large, non-uniform light distribution in which the field intensity of light in the direction of a resonator of the laser, i.e., the resonator direction, is higher on the laser light emission end face (AR) side and is lower on the rear end face (HR) side is exhibited. In this case, more carriers injected into an active layer are consumed in the vicinity of the laser light emission end face (AR), whereas few carriers injected into the active layer are consumed in the vicinity of the rear end face (HR). On the other hand, since current is injected such that the density thereof is maintained constant in the resonator direction, non-uniform carrier concentration distribution called “spatial hole burning (SHB)” occurs in the resonator direction. Due to the influence of SHB, sufficient gain cannot be obtained. As just described, in the conventional high-power laser having the straight stripe structure, if the reflectance at the light emission end face is excessively decreased to provide a high slope efficiency, and such an excessive decrease extremely increases the difference in reflectance between the light emission end face and the rear end face, the field intensity becomes non-uniform. As a result, the gain is decreased, and the slope efficiency is lowered. This leads to limitations on light output, and it is difficult to obtain a light output of equal to or greater than one watt.
For the foregoing reasons, in a conventional laser described in Japanese Unexamined Patent Publication No. 2001-358405 (hereinafter referred to as “Patent Document 1”), a tapered stripe structure in which the width of a waveguide gradually increases in a resonator direction as illustrated in FIG. 32 is used to reduce significant the non-uniform field intensity distribution of light in the resonator direction, thereby reducing occurrence of SHB.
In this case, even if reflectance at a light emission end face is lowered for the purpose of improving a slope efficiency, and the difference in reflectance between the light emission end face and a rear end face extremely increases accordingly, saturation of gain due to occurrence of SHB in the resonator direction can be reduced.
Thus, even if reflectance at a light emission end face is set at equal to or lower than 0.1% to provide a high slope efficiency, and a mirror loss increases accordingly, there is no saturation of gain caused due to SHB, and a high slope efficiency can be provided.
Moreover, since the field intensity of light is not concentrated at the light emission end face of a resonator, occurrence of COD at the light emission end face can be reduced.
The tapered stripe structure is used not only for reducing occurrence of SHB, but also for changing the shape of the distribution of guided light propagating in the waveguide and for cutting off laser light in high-order transverse modes and amplifying and outputting only laser light in a fundamental transverse mode. In addition to the stripe structure described in Patent Document 1, stripe structures described in Japanese Unexamined Patent Publication No. H05-067845, Japanese Unexamined Patent Publication No. 2002-280668, Japanese Unexamined Patent Publication No. 2005-012178, and U.S. Pat. No. 6,317,445 have been reported.